Much of the lifeblood of the world economy flows through pipeline transportation systems. Large volumes of products as diverse as petroleum and liquid hydrocarbons, natural gas, propane, and slurries of solids such as granulated coal and minerals such as copper and iron are constantly being transported between production sites and processing and consumption sites over long distances. These pipelines range generally between 12 inches and 60 inches in diameter and extend to thousands of miles in length. In addition, there are curves and bends along the pipeline with radii of curvature of generally about three times the pipeline diameter, though tighter bends are possible. Usually constructed of metal, in particular, ferrous metals, pipelines are susceptible to damage and other defects which affect the integrity of the system. The result can be a failure which threatens life and property, serious environmental damage, disruptions to both local and distant economies, and loss of the product being transported. The further result can be reduced public confidence in this efficient and economic means of transporting materials with possible public opposition to the growth of such means.
To minimize the risk of failure, pipelines are closely monitored and inspected. One inspection method utilizes pipeline inspection apparatus which are inserted into the pipeline and move through the pipeline generally, but not exclusively, via the flowing material in the pipeline. Such pipeline inspection apparatus may comprise magnetic components to induce magnetic flux (commonly illustrated by lines) within the pipeline wall. The magnetic flux naturally enters the metal wall of the pipeline and distributes evenly to produce a full volumetric inspection. Anomalies or defects in the wall of the pipeline tend to disrupt the uniform flow of the flux and create a leakage of magnetic flux which can then be detected by sensors, generally within the apparatus itself. This inspection methodology is known as magnetic flux leakage (“MFL”). MFL-based apparatus have the capability of addressing, with gas pipelines for example, nearly all the threats listed in ASME B31.8S except incorrect operation and incorrect equipment.
Other inspection methods include, for example, inducing eddy currents in the pipeline via the placement of an auxiliary magnetic pole and relative movement of the inspection apparatus and the pipeline wall. See, e.g., U.S. Pat. No. 5,751,144 to Weischedel (“Weischedel”). Such methods generate circumferential currents that are best employed when attempting to detect axial cracks. As described in Weischedel, one of the “necessary conditions” for reliable detection is the induction of “substantial eddy currents so that eddy current changes representative of structural faults can be readily detected.” To properly induce such eddy currents, the inspection apparatus must first include a small, relative to the two main poles, “auxiliary pole” which “has the same magnetic poling as one of the primary poles” and the apparatus must be moving at a rate in excess of, generally, about four miles per hour (“mph”) relative to the pipeline wall to generate measurable and reliable eddy current signals. In addition, inspection apparatus which induce and rely upon eddy currents must necessarily induce only magnetically saturated states to reduce permeability and allow the eddy currents to penetrate the entire pipeline wall thickness. Sensors are placed to detect maximum eddy current. In contrast, MFL-based apparatus rely upon MFL for detecting changes in the magnetic field associated with corrosion and mechanical damage. Eddy currents, which can interfere with the MFL signals, are minimized by selecting a suitable velocity for the apparatus and by positioning sensors where any spurious eddy currents are at a minimum and the magnetic field is most constant.
Each implementation of MFL technology typically focuses on a subset of pipeline wall anomalies that affect pipeline integrity. And, there are varying levels of success, or sensitivity, for each implementation and not all implementations will provide sufficient information for detailed defect assessments.
MFL-based apparatus for detecting corrosion commonly use high magnetic fields to saturate the pipeline material. Such high magnetic field-based apparatus help suppress noise due to local stress variations and changes in the microstructure of the metal. At metal-loss defects, such as those caused by corrosion, an increased amount of magnetic flux attempts to flow through the remaining material, but some flux leaks from the pipeline wall. In addition, a second phenomenon causes even more flux to leak. In magnetically saturated materials, an increase in flux causes the flux-carrying capability (permeability) to decrease. This double effect of increased flux and decreased flux-carrying capacity results in significant flux leakage at such defects.
Stress and material variations can also change the flux-carrying capacity of magnetic materials such as metal pipe. A local decrease in flux-carrying capacity causes leakage similar to that resulting from metal-loss defects. A local increase in flux-carrying capacity causes a decrease in flux leakage relative to the nominal, magnetic field level. For example, for tensile stresses, the overall flux levels in the pipeline increase. For compressive stresses, such as cold-worked areas, the flux levels decrease. It is known, for example, that such flux density variations between tensile stresses and compressive stresses are small for magnetic field levels greater that about 80 Oersted and particularly for magnetic field levels greater than about 120 Oersted. As will be appreciated by one skilled in the art, however, these values may vary by up to 20 percent with pipeline wall chemical composition, grain structure, and fabrication methods. As discussed above, most MFL-based apparatus for corrosion are designed to operate above these levels to reduce stress noise. To detect stress changes in the pipeline wall, however, the magnetic field must be at lower, unsaturated levels, typically about between 50 and 70 Oersted. Unfortunately, field levels in this range can produce results that are difficult to interpret because they can be affected by corrosion, stresses, and changes in material composition. For example, stress damage is often accompanied by metal loss due to corrosion. While current commercially-available low field strength MFL-based apparatus can be used to detect stresses and material variations using fields in the range of about 50 to 70 Oersted, noise and signal processing and interpretation to properly detect anomalies is difficult.
Thus, two magnetic field levels can improve the detection and assessment of pipeline anomalies. The high magnetic field employed in most inspection apparatus detects and sizes metal loss such as corrosion. A low magnetic field must also be applied to detect the metallurgical changes caused by mechanical damage (e.g., from excavation equipment). It is known, therefore, to utilize an approach using more than one apparatus sent separately through the pipeline. This approach, however, is expensive and disruptive to the operation of the pipeline. A single apparatus having two separate sets of magnetizers, while a technically feasible way to apply dual magnetization technology, results in an increase in the size of the apparatus to unacceptable lengths. In addition, when the two magnetizers are placed in close proximity to one another, a magnetic interaction occurs which distorts the constant magnetizing fluxes. In this case, the lower field slightly increases the high field, but, more importantly, the high field distorts the zone of constant magnetization for low magnetization levels. Furthermore, this effect is more pronounced at higher apparatus velocities.
There is, therefore, a need for a single MFL-based apparatus and method which is capable of detecting metal loss such as corrosion as well as stresses such as those caused by mechanical damage.